The present invention relates to friction clutches in general, and more particularly to improvements in friction clutches with two friction clutch units. Such clutches can be used in automotive vehicles to perform several functions, for example, to connect or disconnect the input element of a change-speed transmission with the output element of the internal combustion engine as well as to transmit or terminate the transmission of torque to a piece of auxiliary equipment, e.g., to a power take-off.
A friction clutch of the above outlined character has two friction discs, one for each unit, and a discrete pressure plate for each friction disc. A spring (e.g., a dished spring) is installed between the pressure plates and serves to urge each pressure plate against the respective friction disc. A common housing is provided for the two units of the clutch, and a surface of the housing is adjacent to and engages with one of the friction discs. The other friction disc is adjacent to a surface of the flywheel which is driven by the internal combustion engine of the vehicle. The housing of the clutch is attached to and rotates with the flywheel. The housing transmits torque to both pressure plates and carries the means for disengaging the two units when the need arises. In conventional clutches, the disengaging means comprises a set of release levers which are pivotally mounted on the housing and can be actuated to move one of the pressure plates away from the associated friction disc. A second set of levers (hereinafter called changeover levers) is employed to act between the housing on the one hand and the two pressure plates on the other hand in such a way that they compel the other pressure plate to move away from the adjacent friction disc when the one pressure plate is disengaged by the release levers.
A drawback of presently known clutches of the above outlined character is that the auxiliary levers contribute excessively to the initial and maintenance cost of the clutches. Reference may be had to German Offenlegungsschrift No. 2,264,254 which discloses metallic changeover levers each of which must be machined with a very high degree of precision. Thus, each such lever must be treated in one or more complex material removing machine tools. The changeover levers are mounted for pivotal movement on shafts which are provided on or secured to the housing of the clutch. The solid and bulky changeover levers are heavy so that they cannot be used in clutches which operate at high rotational speeds unless the supports for such levers are designed with a requisite degree of sturdiness. This, in turn, contributes to the cost and weight of the clutch. In addition to the aforedescribed drawbacks, changeover levers of the type disclosed in the German publication necessitate the provision of other precision-finished parts, such as the shafts on which the levers turn with reference to the housing of the clutch. The changeover levers must turn with a minimum of clearance, i.e., their bearing surfaces as well as the surfaces of the shafts must be machined with utmost precision.
FIG. 3 of the aforementioned German publication shows a simplified changeover lever which exhibits a number of drawbacks. Those portions of the lever which cooperate with the pressure plates are offset with reference to each other, as considered in the axial direction of the housing; this entails a sliding movement between the lever and the pressure plates when the clutch is actuated so that the wear upon the lever and the adjacent surfaces of the pressure plates is very pronounced.